The present invention relates to recent improvements in the handling and utilization of potentially harmful substances, such as organic solvents, to improve environmental safety associated with their use. More particularly, it relates to a new and improved apparatus and method for removing residues and impurities from a circulating solvent flow circuit including a self-scrubbing thermosyphon distillation subassembly for separating purified solvent for return to the flow circuit and a controlled waste collector subassembly for overflowing concentrated residues and impurities into an easy-to-service container.
Recently, the number of kinds of organic solvents commercially employed in a variety of end use applications has increased. Hydrocarbons, halohydrocarbons, fluorocarbons and aromatic solvents are now employed in a number of cleaning applications. Dry cleaning operations for clothing, sometimes referred to as chemical cleaning in Europe, may be the most widespread use for organic solvents of this type. Other uses include cleaning operations for electronic equipment, e.g., refluxing of printed circuit boards. Moreover, various metal cleaning and parts washing apparatus employ special solvents. Some metal cleaning operations prepare surfaces to receive paint or other coatings. Alternatively, solvents may be used to soften and remove unwanted paint and coatings from various substrates such as, for example, in spray guns or other automatic painting equipment.
Concurrently, an awareness of the possibility of environmental damage caused by careless handling and disposal of these organic solvent materials has also increased. Concerns over contamination of ground water supplies caused by the leaking and spilling of used or contaminated solvent onto the ground has long been a concern. More recently, it has been suggested that uncontrolled release of some or all of these organic solvent vapors into the atmosphere may be harmful. Release of some materials may cause a health hazard by increasing the ozone content of the lower atmosphere. Other materials are believed to cause an increased degradation of the ozone layer in the upper atmosphere. Various chlorinated fluorocarbons have been seriously implicated in this regard and a reduction and eventual ban of their use has been recommended by international agreements, such as the Montreal Protocol. Additional health concerns have been noted with respect to the possible carcinogenicity of low molecular weight hydrocarbon and halo-hydrocarbon materials.
The environmental, health and safety concerns surrounding a common end use application for controlled organic solvents is well illustrated by the case of a small to moderately-sized commercial dry cleaning operation. In a commercial dry cleaning operation, customers clothes are collected at a storefront location. The clothes are washed in cleaning machines located on the premises or at a separate location. The clothes are tumbled in an amount of a cleaning solvent which is periodically circulated in batches into the cleaning machines from a reservoir. After agitation of the clothes in the presence of the fluid takes place, the cleaning fluid, usually 1,1,1-trichloroethane (prohibited as of 1995), tetrachloroethylene, or 1,1,2-trichloro-1,2,2-trifluoroethane (known as CFC-113 or Freon.RTM.113 (DuPont)) is drained into a collecting basin associated with the clothes washing units. Additional fluid remaining on the clothes is extracted by spin cycling and this additional solvent is added to the collecting basin.
The used, collected cleaning solvent is then returned to the reservoir and is recycled and continuously used from the reservoir until it is time to replenish the solvent supply, in whole or in part. Frequently, during a portion of the dry cleaning solvent circulation cycle, the solvent may be subject to treatment for removal of solids or particulate matter, namely, lint, grit and the like by means of filters and for removal of water or the like by liquid separators.
In the course of repeatedly re-using the dry cleaning solvent, the solvent per se is not degraded or damaged but becomes contaminated with non-volatile residues and impurities of varying kinds. Normally, the kinds of impurities miscible with a cleaning solvents are oily residues from a number of sources, such as body fluids, air pollution and various foods and beverages. As the concentration of these non-volatile residues builds up in the solvent, the solvent becomes increasingly less effective at cleaning. Eventually, the entire batch of solvent must be removed containerized and transported to a recycling or disposal center.
At this point a number of disadvantages and environmental concerns may be identified. It is now well known that every effort should be made to reduce, reuse and recycle potentially harmful materials. Reuse of the solvents is currently practiced, but after a given level of contamination is reached, re-conditioning or replacement of the solvent is required. Recycling of the dry cleaning fluids however also presents special additional risk factors to be considered. During recycling care should be taken to reduce or eliminate the risk of spillage and/or of exposing volatile organic solvents to humans and the atmosphere. Removing the solvent from the dry cleaning circuit and placing it into containers for shipment to a treatment center introduces the added possibility for spillage and exposure to vapors. Furthermore, transporting the containers to a remote solvent reconditioning center increases the risk of spillage and exposure because of the risk of highway or rail accidents. For these reasons providing a controlled, on-site solvent regeneration system is clearly more desirable than shipping new solvent to a location and shipping used solvent away from the same location on a frequent basis.
Prior art equipment for separating re-usable solvent fractions from non-volatile residues and impurities introduced into the solvent during use have generally been based on distillation equipment and has not been altogether satisfactory. Prior art distillation equipment is generally involves heating with high pressure steam by means of a jacket located in a wall of the distillation vessel. Sometimes a tube bundle is used as a heat source. Frequently, a motor-driven agitator designed to scrape the walls is provided. The prior art scraper systems are generally ineffective to prevent deposition of resinous coatings on the heat transfer surfaces. Once deposition occurs, heat transfer is impeded or halted completely. At this point, it becomes necessary to dismantle the distillation equipment to permit the interior of the vessel to be scraped and cleaned exposing workers and the environment to chemical vapors.
Prior art batch stills usually comprise manually operated apparatus requiring undesirably high amounts of solvent handling. In the case of stills used for dry-cleaning solvents, direct heating without agitation is typical. In the hands of an unskilled operator with little or no distillation background, these stills are frequently operated at overheating temperatures. Overheating causes residue decomposition which may also produce and introduce secondary contaminants into the air or solvent which are possibly toxic. Moreover, overheating causes excess carry-over of residues resulting in unsatisfactory separations.
In one prior art still equipped with a wall scraping stirrer/agitator and adapted for solvent flow rates of as high as 500 gallons per hour, the heating surfaces become coated and ineffective after only about one hour.
In view of the failure of the prior art to provide effective devices and methods for providing environmentally-safe cleaning and regeneration/separation of re-usable solvent from non-volatile residues (NVRs) on the premises, it is an object of this invention to provide improved apparatus and methods for accomplishing on the premises cleaning and regeneration/separation of re-usable solvent from non-volatile residues (NVRs).
It is another object of this invention to provide a new and improved still apparatus for concentrating NVRs and other impurities for batch collection and removal in easy-to-service containers.
It is a further object of the present invention to provide a new and improved apparatus for regenerating clean solvent including a self-scrubbing thermosyphoning heat transfer assembly which does not become dirty and retains its heat transfer efficiency, even after prolonged periods of continuous operation and even when exposed to NVR concentrations in excess of 40%.
It is still another object of the present invention to provide a new and improved solvent distillation apparatus adapted to be placed in a parallel in-line relationship with a circulating solvent stream to extend the useful life of a given quantity of solvent cycling through the solvent stream on the premises of use.
It is a further object of the present invention to provide an on-the-premises solvent recovery distillation plant employing a new and improved self-cleaning heating system which may be maintained without disturbing the remaining portions of the distillation apparatus to reduce or eliminate exposure of personnel to chemicals.
It is still another object of the present invention to provide an apparatus and method for conducting solvent distillation under conditions mild enough to avoid decomposition of solvent and solvent contents, so that non-volatile residues and impurities stripped from the solvent may be recovered intact, should. The NVRs be known or discovered to be separately useful or reusable.